[xj-s] Breaks + overheating = soft?

[sean.straw+Jaguar@mail.professional.org (Sean Straw)]

Today I decided to head out for a nice cruise along the coast (locally), up
Mt. Tamalpaias (just north of San Francisco), and down along some winding
roads - any locals on the list might recognize "Fairfax-Bolinas Road".

Anyway, this is a six mile stretch of downhill winding road - lots of
switchbacks, and some fairly constant break application.

I'm driving along and all is well, but when I get to the bottom at a
midpoint (about where it basically levels out from being a downhill thing,
but lots of turns remain), I note a nasty "burning" smell, and the breaks
are soft.

I pull out and check under the hood - no leaks at the master cylinder, and
the reservoir is full.  Repeated pressing of the break pedal "pumps it up",
as if there were air in the line.  I back it up after much trouncing of the
pedal, and check for leaks - none found.

I decide that it must be a thermal thing, and carefully drive the rest of
the trip home (winding roads, but not too steep of a grade any more).  Over
the course of a few miles, I note that the responsiveness of the breaks has
returned somewhat, but it is still A LOT softer than it was when I set out
earlier in the evening (started the trip around 5pm - the views and sunset
up here are gorgeous).

Can anyone shed any light on this?  I'll be running some tests (and
checking for fluid) in the morning after it has thoroughly cooled, and do a
visual of the discs.

---                             <http://jaguar.professional.org/>
Sean Straw                      '88 Jaguar XJSC 5.3L V12
Marin County, California        '69 Buick GranSport 455 V8

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